Via chains for defect localization

ABSTRACT

Method form via chain and serpentine/comb test structures in kerf areas of a wafer. The via chain test structures comprise a first via chain and a second via chain in a first kerf area. The via chain test structures are formed such that geometrically shaped portions of the first via chain and geometrically shaped portions of the second via chain alternate along the length of the first kerf area. The methods perform relatively low (first) magnification testing to identify a defective geometrically shaped portion that contains a defective via structure. The methods then perform relatively high (second) magnification testing only within the defective geometrically shaped portion. The first magnification testing is performed at a lower magnification relative to the second magnification testing.

BACKGROUND

The embodiments herein relate to the design of via chains andserpentine/comb testable structures, and more specifically, tostructures and methods that save time and are less destructive whentesting via structures.

An integrated circuit (IC) is a semiconductor device containing manysmall, interconnected components such as diodes, transistors, resistors,and capacitors. These components function together to enable the IC toperform a task, such as control an electronic device or perform logicoperations. ICs are found in computers, calculators, cellulartelephones, and many other electronic devices.

ICs and other semiconductor devices are fabricated on small rectangles,known as “dies,” which are filled with multiple layers of thecomponents, such as transistors, resistors, and capacitors, during thefabrication process. The connections between the layers are known asvias. A via is a metal interconnect coupled between two planarconductive layers in a semiconductor die. Multiple vias may be coupledtogether in what may be referred to as a “via chain” connecting oneconductive region in an IC to another conductive region.

A manufacturing error in one of the components mentioned above mayrender an IC or semiconductor device incapable of functioning properly.For example, consider a memory device containing several ICs. If atransistor within one of the ICs fails to function properly, the memorydevice may produce memory errors. Vias are also subject to manufacturingerrors. When a manufacturing error occurs in a via, the via may notconduct properly and thus may prohibit an IC from functioning correctly.For instance, an open via or a partially open via may prohibit a devicefrom functioning as designed. An open via may have a high or infiniteresistance, and a partially open via may have a higher than averageresistance. Therefore, testing via structures is a fundamental aspect ofIC production. The embodiments also relate to in-plane structures aswell.

SUMMARY

An exemplary method herein forms via chain test structures in kerf areasof a wafer. The via chain test structures comprise a first via chain anda second via chain in a first kerf area. The via chain test structuresare formed such that geometrically shaped portions of the first viachain and geometrically shaped portions of the second via chainalternate along the length of the first kerf area.

The methods herein perform relatively low magnification (sometimesreferred to herein as “first magnification”) failure analysis toidentify a defective geometrically shaped portion that contains adefective via structure. The methods then perform relatively highmagnification (sometimes referred to herein as “second magnification”)defect localization only within the defective geometrically shapedportion. The first magnification testing is therefore performed at alower magnification relative to the second magnification testing. Thissaves time and is less destructive because the relatively highmagnification defect localization is slower and potentially moredestructive to the kerf areas than the relatively low magnificationdefect localization.

Another exemplary method herein simultaneously manufactures integratedcircuit chips on a wafer and forms via chain test structures in kerfareas of the wafer. The kerf areas of the wafer are located between theintegrated circuit chips. The methods herein test the via chain teststructures. After testing the via chain test structures, the methodsherein divide the wafer to separate the integrated circuit chips fromeach other in a process that destroys the kerf areas.

The via chain test structures comprise a first via chain and a secondvia chain in a first kerf area. The via chain test structures are formedsuch that geometrically shaped portions of the first via chain andgeometrically shaped portions of the second via chain alternate alongthe length of the first kerf area.

The failure analysis process performs relatively low (first)magnification defect localization to identify a defective geometricallyshaped portion that contains a defective via structure. The failureanalysis then performs relatively high (second) magnification testingonly within the defective geometrically shaped portion. The firstmagnification testing is therefore performed at a lower magnificationrelative to the second magnification testing. This saves time and isless destructive because the relatively high magnification localizationtakes longer and may be more destructive to the kerf areas than therelatively low magnification localization.

A wafer structure according to embodiments herein includes integratedcircuit chips and kerf areas located between the integrated circuitchips. Also, via chain test structures are located in the kerf areas.The via chain test structures include a first via chain and a second viachain located in a first kerf area. Geometrically shaped portions of thefirst via chain and geometrically shaped portions of the second viachain alternate along a length of the first kerf area. The first viachain comprises a first electrical circuit beginning and ending at afirst location within the first kerf area, and similarly the second viachain comprises a second electrical circuit that also begins and ends atthe first location within the first kerf area.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments will be better understood from the following detaileddescription with reference to the drawings, which are not necessarilydrawing to scale and in which:

FIG. 1 is a top view schematic diagram illustrating a wafer according toembodiments herein;

FIG. 2 is a top view schematic diagram illustrating via chains accordingto embodiments herein;

FIG. 3 is a top view schematic diagram illustrating via chains accordingto embodiments herein;

FIG. 4 is a top view schematic diagram illustrating via chains accordingto embodiments herein;

FIG. 5 is a top view schematic diagram illustrating via chains accordingto embodiments herein;

FIG. 6A is a side view schematic diagram illustrating via chainsaccording to embodiments herein;

FIG. 6B is a perspective view schematic diagram illustrating via chainsaccording to embodiments herein;

FIG. 6C is a perspective view schematic diagram illustrating via chainsaccording to embodiments herein;

FIG. 7 is a perspective view schematic diagram illustrating via chainsaccording to embodiments herein;

FIG. 8 is a top view schematic diagram illustrating via chains accordingto embodiments herein;

FIG. 9 is a top view schematic diagram illustrating via chains accordingto embodiments herein;

FIG. 10 is a top view schematic diagram illustrating via chainsaccording to embodiments herein;

FIG. 11 is a top view schematic diagram illustrating via chainsaccording to embodiments herein;

FIG. 12 is a flow diagram illustrating embodiments herein;

FIG. 13 is a top view schematic diagram illustrating via chainsaccording to embodiments herein;

FIG. 14 is a top view schematic diagram illustrating via chainsaccording to embodiments herein;

FIG. 15 is a top view schematic diagram illustrating via chainsaccording to embodiments herein; and

FIG. 16 is a top view schematic diagram illustrating via chainsaccording to embodiments herein.

DETAILED DESCRIPTION

As mentioned above, electrical test and failure analysis of via and/orsnake structures are a fundamental aspect of IC production. Theembodiments herein provide structures and methods that save time and maybe less susceptible to potential damage when localizing defects infailure analysis.

Vias in a semiconductor device may be tested by measuring the resistanceof via test chains. This technique identifies via chains containing openvias and some partially open vias by their high resistance. The testmodule may be located in the kerf region surrounding the semiconductordie. The kerf regions are areas where semiconductor wafer will be cut toseparate individual semiconductor dies when the fabrication process iscomplete. Semiconductor test structures in semiconductor dies or kerfregions contain a plurality of vias. Vias are conductors thatelectrically connect one layer of an integrated circuit to anotherlayer. Vias may be connected together to form a via chain. The via chainconnects layers of components in the semiconductor test structure. Eachlayer contains a plurality of conductive plates. Each plate couples toat least one other plate by one of the vias.

It is relatively easy to localize defects in macros with very smallareal coverage; however, in order to achieve defect density processtargets, it is often necessary to make extremely large arrays withlinks. It is natural that in these larger via chains it would bedifficult to locate defects (localize defects).

Referring now to the drawings, FIG. 1 is a top-view schematicillustration showing a wafer 100 structure that includes integratedcircuit chips 102 and kerf areas 104 located between the integratedcircuit chips 102. FIG. 2 is also a top-view schematic illustration ofan enlarged portion of the structure shown in FIG. 1. More specifically,FIG. 2 illustrates via chain test structures 106 that are located in akerf area 104.

FIG. 3 is a top enlarged view of the via chain test structures 106 shownin FIG. 2. As shown in FIG. 3, the via chain test structures 106 includea first via chain 110 and a second via chain 120 located in one of thekerf areas, which is arbitrarily referred to herein as a “first” kerfarea 104. As shown in the conceptual drawing in FIG. 4, thegeometrically shaped portions 112 of the first via chain 110 andgeometrically shaped portions 122 of the second via chain 120 alternatealong a length of the first kerf area 104. The same structure, where thegeometrically shaped portions 112 of the first via chain 110 andgeometrically shaped portions 122 of the second via chain 120 alternatealong a length of the first kerf area 104, is shown somewhat moreschematically in FIG. 5.

Thus, as shown here there can be two types of via chains: i) via chains,which alternate, in squares (& other shapes), by type, across two ormore layers of the chip; and ii) serpentine/comb structures whichalternate in type or instance in the same plane of the chip. These arereferred to as “comb” structures because they are shaped like the teethof a comb. For example, considering FIG. 4, above, the via chains, whichalternate, in squares (& other shapes), by type, across two or morelayers of the chip is shown where the path of the squiggly linesrepresent two different via chains. The serpentine/comb structures whichalternate in type or instance in the same plane of the chip are shownwhere the one color line is, say RX (AKA, active area) and the othercolor line is PC (AKA gate).

Despite the conceptually different ways FIGS. 3-5 illustrate thestructure, in the embodiments herein, the first via chain 110 iselectrically insulated from the second via chain 120. Further, the firstvia chain 110 comprises a first electrical circuit beginning and endingat a first location 108 within the first kerf area 104, and similarlythe second via chain 120 comprises a second electrical circuit that alsobegins and ends at the first location 108 within the first kerf area104.

While a single beginning/ending location 108 is illustrated in FIG. 4,as would be understood by those ordinarily skilled in the art, each viachain circuit could have a beginning positioned at a different locationthan the ending. Therefore, in FIG. 5 a beginning location 108 and anending location 118 are illustrated for the second via chain 120circuit. In order to avoid clutter in FIG. 5, some additional wiring ofthe first via chain 110 circuit has intentionally been omitted. However,as is illustrated in FIGS. 3-5, each via chain circuit includes abeginning, an ending, and a plurality of geometrically shaped groupingsof via structures that alternate with other geometrically shapedportions of other via chains.

While the foregoing examples have discussed only two via chains, thoseordinarily skilled in the art would understand that the number of viachains is not limited and any of the embodiments herein could include alarge number of via chains, but such large numbers are not illustrated,simply for ease of illustration and to simplify understanding. In oneexample shown in FIGS. 6A-6C, three different via chains 110, 120, 130are shown in a cross-sectional schematic (FIG. 6A); perspectivecross-sectional schematic (FIG. 6B); and perspective elevated-viewschematic (FIG. 6C).

More specifically, FIG. 6A illustrates three different conductor layers(M_(x), M_(x+1), and M_(x+2)) within an integrated circuit structure,and three different via layers (V_(x), V_(x+1), and V_(x+2)) thatconnect the different conductor layers together. Therefore, as shown inFIG. 6A, the first via chain 110 includes vias V_(x+2) that connectconductor layer M_(x+3) and conductor layer M_(x+2). Similarly, thesecond via chain 120 includes vias V_(x) that connect conductor layerM_(x+1) and conductor layer M_(x). Also, the third via chain 130includes vias V_(x+1) that connect conductor layer M_(x+2) and conductorlayer M_(x+1).

In order to avoid cluttering the drawings, FIGS. 6A-6C only illustrate asingle geometrically shaped portion of each of the via chains 110, 120,130; however, those ordinarily skilled in the art would understand thateach of the via chains 110, 120, 130 illustrated in FIGS. 6A-6C, iselectrically insulated from the other via chains, and each via chaincomprises an electrical circuit having multiple ones of thegeometrically shaped portions. Further, each of the via chains isconnected to wiring that allows each via chain to be tested from asingle location (or from two locations).

While FIGS. 6A-6C illustrate that the via chains can be on differentlevels within the integrated circuit structure, FIG. 7 is an elevatedperspective view illustrating the first via chain 110 and the second viachain 120 positioned within the same levels of an integrated circuitstructure. FIG. 7 is therefore very similar to the structure shown inFIGS. 3-5, discussed above, and illustrates such structures inperspective view.

The geometrically shaped portions 112, 122 can be of any desired shapeincluding, but not limited to triangles, rectangles, pentagons,hexagons, heptagons, octagons, circles, ovals, etc., and a few of theseshapes are illustrated in FIGS. 8-11. Note that in FIGS. 8-11, only thegeometric portions 112, 122 containing multiple via structures areillustrated, and all wiring and other associated structures are omittedin order to avoid clutter. Therefore, FIG. 8 illustrates squaregeometric portions 112, 122 that can be positioned in a checkerboardarrangement. Similarly, FIG. 9 illustrates triangle geometric portions112, 122, FIG. 10 illustrates pentagon geometric portions 112, 122, andFIG. 11 illustrates hexagon geometric portions 112, 122. While thedrawings illustrate a limited number of shapes, those ordinarily skilledin the art would understand that many more shapes could be utilized forthe geometric portions 112, 122 and that the shapes shown here aremerely illustrative and do not limit the embodiments to these specificshapes.

The structures shown above are used to save time and reduce destructiveeffects when testing and when performing defect localization viastructures. More specifically, as shown in flowchart form in FIG. 12, initem 200 an exemplary method herein simultaneously manufacturesintegrated circuit chips on a wafer and forms via chain test structuresin kerf areas of the wafer. The methods herein test the via chain teststructures in item 202-204. After testing the via chain test structures,in item 206 the methods herein divide the wafer to separate theintegrated circuit chips from each other in a process that destroys thekerf areas.

As shown above, the via chain test structures 106 comprise a first viachain 110 and a second via chain 120 in the kerf area 104. The via chaintest structures 110, 120 are formed such that geometrically shapedportions 112 of the first via chain 110 and geometrically shapedportions 122 of the second via chain 120 alternate along the length ofthe kerf area 104.

The testing process takes advantage of this structure and first performsrelatively low power, low magnification testing (sometimes referred toherein as “first magnification testing”) in item 202 to quickly andnon-destructively identify whether any defective vias are present and,if they are, to identify at least one defective geometrically shapedportion that contains a least one defective via structure.

One benefit here is in defect localization in failure analysis.Therefore, the defect localization may regionalize the area of a defectin an analysis like Focused Ion Beam, Externally Induced VoltageAlteration, or Optical Beam Induced Current. A low magnification scan inthe failure analysis localization (not in “testing”) can quicklyidentify the region of the defect. This has two benefits. i) for alllocalization techniques, it greatly saves analysis time and increasesthe chance that the localization will provide useful information to findthe point defect; ii) in some localization techniques, such as FocusedIon Beam (FIB), the scanning of the image itself can be destructive.Therefore, minimizing the number of high-magnification scans that needto be done provides great benefit in not destroying the sample beforethe defect is found).

In item 204, the testing process then performs relatively high power,high magnification testing (sometimes referred to herein as “secondmagnification testing”) only within the defective geometrically shapedportion(s). The first magnification testing is performed at a lowermagnification relative to the second magnification testing. This savestime and is less destructive because the relatively high magnificationtesting takes longer and is more destructive to the kerf areas than therelatively low magnification testing. By limiting the relatively highmagnification testing to only those geometrically shaped portions thathave been identified as being defective geometrically shaped portions,less of the slower, more destructive testing is preformed, therebysaving time and reducing destruction.

Testing techniques include focused ion beam (FIB) testing, laser voltageimaging, magnetically scanned beam, scanning electron microscopy,optical microscopy, Energy Dispersive X-Ray Spectroscopy (EDS) etc. Inone of these methods, FIB for example, FIB systems operate in a similarfashion to a scanning electron microscope (SEM) except, rather than abeam of electrons and as the name implies, FIB systems use a finelyfocused beam of ions (usually gallium) that can be operated at low beamcurrents for imaging or high beam currents for site specific sputteringor milling.

An exemplary FIB unit generally contains a liquid metal ion source usinggallium (Ga) to generate a Ga ion beam. A lens system focuses the ionbeam to a spot size on the test area of the wafer which is placed on astage. A set of scan coils is placed in the vicinity of the lens system.When energized, the scan coils direct the ion beam to scan over apredetermined area. The output of ion source, lens system focusing, andscan coil actions are controlled by an ion beam control unit.

During FIB testing, images produced show portions of the via scan chainthat are electrically conductive as having a contrast relative to thoseportions that are not electrically conductive. Therefore, during lowmagnification testing, each geometrically shaped portion of each scanchain will either show contrast or not show contrast.

For example, in FIG. 13 (which is the same structure shown in FIG. 8)the image produced by low magnification FIB testing shows some of thegeometrically shaped portions 122 of the second via chain 120 havingcontrast (brighter color) and other not having the contrast.Specifically, if testing proceeded from the top left down toward thebottom right, geometrically shaped portion 124 is the firstgeometrically shaped portion that does not show contrast. This indicatesthat geometrically shaped portion 124 is a defective geometricallyshaped portion (as would be indicated during item 202 in FIG. 12).Similar testing can be done in a different order, such as from bottom totop, etc., (or at different powers) to locate other defectivegeometrically shaped portions.

Then, as indicated in item 204 in FIG. 12, relatively high power, highmagnification testing is performed only within the defectivegeometrically shaped portion 124. This is illustrated in FIG. 14, whichis a top view schematic diagram illustrating a small part of the viascan chain within geometrically shaped portion 124. As shown by thechange in contrast at point 126, the via scan chain within geometricallyshaped portion 124 is broken at point 126.

The above-shown features allow for a benefit in localization making useof patterns which allow for a regionalization at low magnificationscanning, which is then followed by high-magnification scanning.However, as shown in FIGS. 15 and 16, the embodiments further allow forthe use of high-magnification images to show precisely where one is whenthe defect is localized. This is of particular benefit in large macrosor in cases where the features of the macro are capable of being damagedby even the low-magnification scanning.

More specifically, such features herein include a grid similar to thatin FIG. 8 where the alternating squares (112, 122) of via chain type 1and 2, except that the X and Y dimensions of the rectangles vary acrossthe arrays as shown in FIG. 15. The dimension of the rectangles maydecrease and increase in several waves as one goes from one edge of themacro to the other.

Identifying marks or changes in the pattern within the squares 112, 122give information as to the coordinates of the square. These features areshown in FIG. 16, where the identifying mark 192 can be made with eithera block of “no pattern” (i.e., dielectric), some other material, or the“alternate” via serpentine type.

The power of the FIB testing can be performed at any power level, whichwill vary with the item being tested. For example, FIB localizationoften involves use of 30 kV Ga. However, the high power, highmagnification testing is at least 10 times higher than the low power,low magnification testing (and can be 10; 100; 1000; 10,000; etc., timeshigher power). This increase in power required for the highermagnification testing is proportionately more destructive on the itembeing tested (e.g., can erode away min-dimension copper). Morespecifically, the focused ion beam can sputter away delicate materialand/or implant Ga+ ions into the sample (creating electrical shorts),causing more damage with more imaging time. This destructive effect isgreater with greater magnification. Therefore, by limiting such highpower, high magnification testing to only the previously identifieddefective geometrically shaped portions, the amount of damage to thekerf region as a whole is dramatically reduced (when compared toperforming high magnification testing on the entire via chain frombeginning to end).

In addition, the process of focused ion beam testing of each individualvia in the high magnification testing is time consuming as therelatively narrow beam used in high magnification testing is directed toeach via sequentially. To the contrary, when performing the lowmagnification testing with a wider beam, each geometrically shapedportion is tested as a single item. In other words, the lowermagnification testing uses a test beam large enough to cover an entiregeometrically shaped portion of via chain, while the high magnificationtesting uses a more narrow beam that is focused down to a size as smallas a single via. Also, between the low and high magnification, the rateof milling of surface material greatly increases, such that it takesgreat skill not to destroy the area of interest with just a few scans.Thus, it takes the same amount of time to test an entire geometricallyshaped portion with a wider focused beam as it does to test a single viaunder higher magnification testing with a more narrowly focused beam. Inthis way, the low magnification testing allows more vias to be testedmore quickly, and the slower high magnification testing is reserved forthe smaller previously located defective geometrically shaped portions.The appearance of the voltage contrast of the low and high magnificationtesting is similar, but in order to find the actual failing link, youwill need to scan a considerable portion of the chain at highmagnification, and there is a risk of destroying your sample before youhave determined the failing link.

Therefore, all geometrically shaped portions can be tested very quickly(e.g., 10; 100; 1000; 10,000; etc., times faster than the time neededfor focused ion beam testing of each individual via in the highmagnification testing) and the slower high magnification testing thatproceeds via-by-via is used only in the previously identified defectivegeometrically shaped portions. By using the slower high magnificationtesting only in the previously identified defective geometrically shapedportions the testing time of the kerf region as a whole is dramaticallyreduced (when compared to performing high magnification testing on theentire via chain from beginning to end).

Further, in very large via chains, it is difficult to find the defectfor marking if it is located in the center of an extremely large array.This problem is solved here because the embodiments herein perform highmagnification testing only in the previously identified defectivegeometrically shaped portions (which can be designed small enough toallow easy location (localization) of the defect).

Thus, the embodiments described herein allow for the localization signalitself to be seen at low magnification as a grid of checkerboard squareswithin the larger matrix. This permits one to drive quickly to the placeof interest, from low magnification to high magnification, simply bydriving to squares. Further, after the defect has been localized(especially in the case of non-destructive techniques) the mark may befound rapidly, by counting off squares.

In addition, terms such as “right”, “left”, “vertical”, “horizontal”,“top”, “bottom”, “upper”, “lower”, “under”, “below”, “underlying”,“over”, “overlying”, “parallel”, “perpendicular”, etc., used herein areunderstood to be relative locations as they are oriented and illustratedin the drawings (unless otherwise indicated). Terms such as “touching”,“on”, “in direct contact”, “abutting”, “directly adjacent to”, etc.,mean that at least one element physically contacts another element(without other elements separating the described elements).

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the embodiments.As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the embodiments has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the embodiments in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the embodiments. Theembodiment was chosen and described in order to best explain theprinciples of the embodiments and the practical application, and toenable others of ordinary skill in the art to understand the embodimentsfor various embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method comprising: forming a first via chainand a second via chain in a first kerf area of a wafer such thatgeometrically shaped portions of said first via chain and geometricallyshaped portions of said second via chain alternate along a length ofsaid first kerf area; performing first magnification testing to identifya defective geometrically shaped portion, said defective geometricallyshaped portion containing a defective via structure; and performingsecond magnification testing only within said defective geometricallyshaped portion, said first magnification testing being performed at alower magnification relative to said second magnification testing, saidsecond magnification testing being slower and more destructive to saidkerf areas than said first magnification testing.
 2. The methodaccording to claim 1, said first via chain comprising a first electricalcircuit beginning and ending at a first location within said first kerfarea, and said second via chain comprising a second electrical circuitbeginning and ending at said first location within said first kerf area.3. The method according to claim 1, said first via chain beingelectrically insulated from said second via chain.
 4. The methodaccording to claim 1, said testing comprising focused ion beam testing.5. The method according to claim 1, said geometrically shaped portionscomprising rectangles such that said alternating ones of saidgeometrically shaped portions of said first via chain and geometricallyshaped portions of said second via chain form a checker-board pattern insaid first kerf area.
 6. The method according to claim 1, saidgeometrically shaped portions comprising one of: geometric structures,which alternate by type, across two or more layers of said kerf area;and serpentine and comb structures which alternate in type or instancein the same plane of said kerf area.
 7. The method according to claim 1,said geometrically shaped portions of said first via chain andgeometrically shaped portions of said second via chain repeating in saidfirst kerf area to from an array of geometrically shaped portions.
 8. Amethod comprising: simultaneously manufacturing integrated circuit chipson a wafer and forming via chain test structures in kerf areas of saidwafer, said kerf areas of said wafer being located between saidintegrated circuit chips; testing said via chain test structures; andafter testing said via chain test structures, dividing said wafer toseparate said integrated circuit chips from each other in a process thatdestroys said kerf areas; said forming of said via chain test structurescomprising forming a first via chain and a second via chain in a firstkerf area such that geometrically shaped portions of said first viachain and geometrically shaped portions of said second via chainalternate along a length of said first kerf area, said testingcomprising performing first magnification testing to identify adefective geometrically shaped portion, said defective geometricallyshaped portion containing a defective via structure, said testingfurther comprising performing second magnification testing only withinsaid defective geometrically shaped portion, said first magnificationtesting being performed at a lower magnification relative to said secondmagnification testing, and said second magnification testing beingslower and more destructive to said kerf areas than said firstmagnification testing.
 9. The method according to claim 8, said firstvia chain comprising a first electrical circuit beginning and ending ata first location within said first kerf area, and said second via chaincomprising a second electrical circuit beginning and ending at saidfirst location within said first kerf area.
 10. The method according toclaim 8, said first via chain being electrically insulated from saidsecond via chain.
 11. The method according to claim 8, said testingcomprising focused ion beam testing.
 12. The method according to claim8, said geometrically shaped portions comprising rectangles such thatsaid alternating ones of said geometrically shaped portions of saidfirst via chain and geometrically shaped portions of said second viachain form a checker-board pattern in said first kerf area.
 13. Themethod according to claim 8, said geometrically shaped portionscomprising one of: geometric structures, which alternate by type, acrosstwo or more layers of said kerf area; and serpentine and comb structureswhich alternate in type or instance in the same plane of said kerf area.14. The method according to claim 8, said geometrically shaped portionsof said first via chain and geometrically shaped portions of said secondvia chain repeating in said first kerf area to from an array ofgeometrically shaped portions.